1. Field of the Invention
The present invention relates to a channel quality estimation method suitable for being applied to a wireless communication terminal that performs communication using a CDMA (Code Division Multiple Access) method and a receiving apparatus that performs the channel quality estimation processing, and to a technology suitable for being applied to data reception in a wireless communication terminal of a HSDPA method, for example.
2. Description of the Related Art
As one of wireless telephone systems, a high speed downlink packet access (hereinafter referred to as HSDPA) method has been developed to obtain high speed downlink data transmission in a universal mobile telecommunications system (UMTS system) that is a system employing a W-CDMA method which is one of CDMA methods.
The HSDPA method is a communication method employing adaptive modulation to speed up downlink data transmission from a base station to a wireless communication terminal, and on the terminal side, a channel quality of a downlink user data channel is detected (in actuality estimated) and is notified to the base station using uplink.
In channel quality detection processing in related art, first, a signal to noise ratio (hereinafter referred to as SNR) of a downlink pilot channel is obtained, for example. Next, an SNR of a user data channel used in the HSDPA method is obtained from the SNR of the pilot channel. Then, the channel quality is obtained from the SNR of the user data channel.
With respect to the processing of obtaining the channel quality, an explanation is made in detail, in which signal power and noise power are obtained from the following expressions of [Formula 1] and [Formula 2], and the SNR of the pilot channel is calculated from an expression of [Formula 3] using both the power.
                    S        =                                                                        1                                  N                  1                                            ⁢                                                ∑                                      i                    =                    j                                                        N                    1                                                  ⁢                                  pilot_symbol                  i                                                                          2                                    [                  Formula          ⁢                                          ⁢          1                ]                                N        =                              1                          N              1                                ⁢                                    ∑                              i                =                j                                            N                1                                      ⁢                          (                                                                                                            pilot_symbol                      i                                        -                    S                                                                    2                            )                                                          [                  Formula          ⁢                                          ⁢          2                ]                                Pilot_SNR        =                  10          ·                                                    log                10                            ⁡                              (                                  S                  /                  N                                )                                      ⁢                                                  [            dB            ]                                              [                  Formula          ⁢                                          ⁢          3                ]                            where            pilot symbol: pilot channel after demodulation    S: signal power    N: noise power    Pilot SNR: SNR of pilot channel    N1: number of pilot channel symbols used to obtain SNR of pilot channel    i: symbol number of pilot channel    j: initial number of pilot channel symbol number used for obtaining SNR are defined, respectively.
In this manner, the SNR of the user data channel which is a synchronization channel is estimated from the SNR of the pilot channel obtained from the expression of [Formula 3]. An expression of [Formula 4] is provided to calculate a Data SNR which is the SNR of the user data channel.Data—SNR=Pilot—SNR−10·log10(SFpilot/SFdata)−Γ [dB]  [Formula 4]
In this expression of [Formula 4], an SFpilot is a spreading factor (256) of the pilot channel and an SFdata is a spreading factor (16) of the user data channel. Further, Γ is a value of power offset. The power offset value is a difference between power of the user data channel and that of the pilot channel, which is presumed when using the channel quality of the user data channel. In the HSDPA method, a ratio of the spreading factor of the user data channel to that of the pilot channel is 1/16, and processing to make the SNR of the pilot channel into 1/16 is basically performed in the expression of [Formula 4].
The channel quality is obtained from the SNR of the user data channel obtained (estimated) as described above. Specifically, the value obtained from the expression of [Formula 4] is, for example, compared with each of threshold values from A to I in a table shown in FIG. 1, and channel quality values in 31 stages from 0 to 30 are obtained in accordance with magnitude relation to each of those threshold values. In Patent reference 1, there is a disclosure of detecting and informing a channel quality in a wireless telephone system.
[Patent reference 1] Published Japanese Patent Application No. 2003-174485
The following problems are recognized in the channel quality of related art calculated in this manner. That is, a noise component is assumed to be White Gaussian Noise (Additive White Gaussian Noise: AWGN) in the above-described expression of [Formula 1]. In addition, with respect to conversion of the spreading factor SF in the expression of [Formula 4], the noise component is assumed to be the White Gaussian Noise.
In the expression of [Formula 1], an influence of the noise component is removed by taking an average of the pilot channel symbols and the signal power is calculated. However, when there exists such noise component that the average thereof does not become zero, the noise component is not removed and the signal power may not be calculated accurately. Further, it also becomes difficult to accurately calculate the noise power which is obtained from the signal power.
In the conversion formula of the spreading factor SF, the SNR of the user data channel is also assumed to be 1/16 of the SNR of the pilot channel, however, a spreading gain may not be obtained without the premise of the White Gaussian Noise, and the relation thereof may not be obtained either.
Further, although there exists a Synchronization Channel (SCH) in the W-CDMA method that is the system to which the HSDPA method is applied, the synchronization channel does not have an orthogonal relation with other channels. Therefore, when the pilot channel is inversely spread or when the user data channel is inversely spread, the synchronization channel becomes a noise component due to a reason of not being orthogonal. Further, the noise component does not become the White Gaussian Noise but becomes a correlated noise. Accordingly, when an influence of the synchronization channel becomes large, an unexpected effect is given to the calculation results of the above-described expressions of [Formula 1] through [Formula 3] and also a relational expression for converting the spreading factor SF may not be obtained.
A transmission timing of the synchronization channel in the W-CDMA method is explained in FIG. 2, in which a first SCH (first synchronization channel) of 256 chips and a second SCH (second synchronization channel) of 256 chips are transmitted at a predetermined interval (head portion) in each slot.
When such synchronization channels exist, the relation between the SNR of the user data channel and threshold values, which is shown in FIG. 1, in a state in which a white noise is influential in all the noise components differs from that in a state in which a noise component caused by the synchronization channel is influential in all the noise components. If the white noise is influential in the presumption when obtaining the threshold value, there may occur such a problem that the channel quality is not accurately detected when the synchronization channel becomes influential. On the contrary, if the synchronization channel is influential in the presumption when obtaining the threshold value, there may occur such a problem that the channel quality is not accurately detected when the white noise becomes influential.
In the HSDPA method, throughput of a system is raised by adaptively changing a modulation method in response to the channel quality. Therefore, in view of raising throughput of the system, it is important to notify the base station side of an accurate channel quality in any reception condition, however, the accurate channel quality has not necessarily been notified in the system of related art.